Neuroblastoma is the most common and deadly solid tumor in children, but this tumor also has a very high propensity to undergo spontaneous differentiation or regression. Evidence suggests that the Trk family of neurotrophin receptors play a critical role in tumor behavior. Neuroblastomas expressing TrkA are biologically favorable and prone to spontaneous differentiation or regression. In contrast, Neuroblastomas expressing TrkB usually have MYCN amplification and are among the most aggressive and deadly tumors known. These tumors also express the TrkB ligand, resulting in an autocrine survival pathway. Unlike the TrkA-expressing tumors, exposure to ligand promotes survival under adverse conditions, but does not cause differentiation. We are exploring the biological basis for the very different behavior of neuroblastomas expressing these highly homologous neurotrophin receptors. We will also determine the consequences of blocking TrkA versus TrkB expressing tumors with the novel, Trk-targeted tyrosine kinase inhibitor CEP-2563. Our specific aims are: Specific Aim 1. Determine the differences in receptor structure, signaling and gene induction that distinguish the biological effects of TrkA versus TrkB expression in human neuroblastoma cells. Hypothesis: A fundamental difference in receptor structure, leading to differences in signaling. Immediate-early gene induction, and/or interactions with P75 explain the difference in biological behavior of TrkA- and TrkB expressing neuroblastomas. Specific Aim 2. Analyze the consequences of TrkA or TrkB inhibition on neuroblastoma behavior and tumorigenicity. Hypothesis: Inhibition of TrkA will induce apoptosis, and inhibition of TrkB will decrease angiogenesis, tumorigenicity and resistance to chemotherapy. The successful completion of these studies should explain why the biological consequences of expressing TrkA versus TrkB are so different, and what portion of the receptor is responsible for this difference. We will also determine the efficacy of the novel tyrosine kinase inhibitor CEP-2563 in treating TrkA and TrkB expressing tumors. Given the increasing evidence for a role of Trk receptors in a variety of pediatric and adult tumors, these data would argue strongly for the utility of Trk receptor inhibition in therapy, alone or in combination.